Lagarosiphon (Lagarosiphon major)

PROHIBITED MATTER: If you see this plant report it. Call the NSW DPI Biosecurity Helpline 1800 680 244
Also known as: oxygen weed, oxygen plant, curly water thyme

Lagarosiphon is a water weed that forms dense mats under or at the water surface. It reduces food and habitat for fish and other aquatic animals.


How does this weed affect you?

Lagarosiphon forms dense mats that can be several metres thick. It:

  • can dominate freshwater lakes, dams and slow-moving streams
  • stops light penetrating the water
  • reduces water quality and oxygen levels
  • outcompetes native vegetation 
  • reduces habitat and food for fish and other aquatic animals
  • reduces recreational activities such as swimming, fishing and boating
  • can block hydro-electric intakes and irrigation systems.

What does it look like?

Lagarosiphon is a perennial plant that grows underwater. It can be attached to the sediment in the waterway and be fully submerged or free floating.


  • are 5–20 mm long and 2–3 mm wide, tapering at the tip
  • have finely toothed margins
  • usually curve downwards towards the stem (they are straight in low alkaline water which may occur in areas with granite or sandstone bedrock)
  • are alternate and spiral along the stem
  • are closer together at the top of the stem.

Flowers are:

  • 3.5 to 5.5 mm long with clear-white or pale pink petals
  • on a very thin white stalk that floats.

Only female flowers have been found outside its native range. In its native range, lagarosiphon produces male flowers that break off and float to the surface.

Stems are:

  • 3–5 mm in diameter and up to 6.5 m long
  • sparsely branched
  • curved towards the base (J-shaped)
  • easily broken and can be free-floating at the surface. 


  • a thread-like root system branches from the stem, anchoring plants to the bottom.
  • rhizomes (horizontal stems) are found in the sediment and also anchor the plant.

Similar looking plants

The main difference between Lagarosiphon and similar looking plants is that Lagarosiphon leaves spiral up the stem. The other plants have leaves that are clustered in groups around the stem. Similar looking plants include:

  • Elodea (Elodea canadensis) which has whorled leaves to 40 mm long.
  • Egeria (Egeria densa) which has much larger flowers 20 mm diameter and whorled leaves in groups of 4–5.
  • Hydrilla (Hydrilla verticillata) a native plant that has whorled, serrated, almost straight leaves.

Where is it found?

There are currently no naturalised infestations in Australia. However lagarosiphon is sometimes planted in aquariums. During the late 1970s, it was found and eradicated from a few small dams near Melbourne and Newcastle. These infestations may have originated from ornamental plants in aquariums or ponds. All infestations have been eradicated.

Lagarosiphon is native to southern Africa. 

It is a major water weed in parts of Europe and New Zealand. 

What type of environment does it grow in?

Lagarosiphon grows in high mountain streams and ponds. It prefers:

  • clear, still or slow-moving freshwater
  • silty or sandy sediments to anchor in 
  • cooler waters (20°C–23°C with a maximum of 25°C)
  • sheltered sites (from wind, waves and currents)
  • lots of light. 

Lagarosiphon can grow in: 

  • high and low nutrient levels
  • clear water up to 6.5 m deep 
  • murky water up to 1 m deep.

Maps and records

  • Recorded presence of Lagarosiphon during property inspections (Map: Biosecurity Information System - Weeds, 2017-2024)
    These records are made by authorised officers during property inspections under the Biosecurity Act 2015. Officers record the presence of priority weeds in their council area and provide this to the NSW Department of Primary Industries. Records reflect the presence of the weed on the date of inspection.

How does it spread?

Lagarosiphon is not known to produce male flowers, fruit or seed in Australia (or anywhere else outside its native range). 

By plant parts

Plant fragments break off and new roots grow from the nodes on the stem. Water and boats spread the fragments and it can move large distances downstream. Infestations are often first recorded at boat ramps.


CRC for Australian Weed Management (2003). Weed management guide: Lagarosiphon major.

Matthews, J., Beringen, R., Collas, F. P. L., Koopman, K. R., Odé, B., Pot, R., ... & Leuven, R. S. E. W. (2012). Knowledge document for risk analysis of the non-native Curly Waterweed (Lagarosiphon major) in the Netherlands. Retrieved 3/08/2020 from:

PlantNET (The NSW Plant Information Network System). Royal Botanic Gardens and Domain Trust, Sydney. Retrieved 3/08/2020 from:

Sainty, G. R., & Jacobs, S. W. (2003). Waterplants in Australia (No. Ed. 4). Sainty and Associates Pty Ltd.

More information

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Please do not attempt to treat or dispose of this weed yourself. Report this plant if you see it anywhere in NSW by calling the helpline listed at the top of this page immediately.

NSW DPI will lead an initial response for the treatment and disposal of the plant to stop it from spreading

Herbicide options

Users of agricultural or veterinary chemical products must always read the label and any permit, before using the product, and strictly comply with the directions on the label and the conditions of any permit. Users are not absolved from compliance with the directions on the label or the conditions of the permit by reason of any statement made or not made in this information. To view permits or product labels go to the Australian Pesticides and Veterinary Medicines Authority website

See Using herbicides for more information.

Diquat 200 g/L (Reglone®)
Rate: 5 L per megalitre water
Comments: Apply by stem injection below the surface or as a surface spray
Withholding period: Do not use treated water for human consumption, livestock watering or irrigation purposes for 10 days after application. Do not graze or cut sprayed vegetation for stock food for 1 day after application. See label for harvest withholding periods.
Herbicide group: 22 (previously group L), Inhibition of photosynthesis at photosystem I via electron diversion (PSI electron diversion)
Resistance risk: Moderate

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Biosecurity duty

The content provided here is for information purposes only and is taken from the Biosecurity Act 2015 and its subordinate legislation, and the Regional Strategic Weed Management Plans (published by each Local Land Services region in NSW). It describes the state and regional priorities for weeds in New South Wales, Australia.

Area Duty
All of NSW General Biosecurity Duty
All pest plants are regulated with a general biosecurity duty to prevent, eliminate or minimise any biosecurity risk they may pose. Any person who deals with any plant, who knows (or ought to know) of any biosecurity risk, has a duty to ensure the risk is prevented, eliminated or minimised, so far as is reasonably practicable.
All of NSW Prohibited Matter
A person who deals with prohibited matter or a carrier of prohibited matter is guilty of an offence. A person who becomes aware of or suspects the presence of prohibited matter must immediately notify the Department of Primary Industries

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For technical advice and assistance with identification please contact your local council weeds officer.

Reviewed 2020